The present invention relates to a magnetic disk drive unit, more particularly to a type of magnetic disk drive unit in which a screw engagement portion of a carriage for holding a magnetic head is engaged with a screw shaft rotated by motor drive.
FIG. 6 shows a conventional arrangement of a magnetic disk drive unit which carries out transference of a carriage by using a screw shaft.
In this figure, between supports 1 and 2 arranged in a spaced parallel relationship with each other are placed parallel guide shafts 3 and 4, and between the support plates 1 and 2 is also placed a screw shaft 5 which is rotated by the rotation of a motor transmitted through gears 7 and 8. A carriage 9 on which a magnetic head 10 is mounted is supported and guided by guide shafts 3 and 4, and a pin (screw engagement portion) 11 thereof is engaged with a screw portion 5a of the screw shaft 5, so that it is transferred in the radial direction of a magnetic disk (not shown) by the rotation of the screw shaft 5. Numerals 12 denote springs for pressing the pin 11 of the carriage 9 against one of walls forming the root of the screw portion 5a.
In the arrangement described above, the screw shaft 5 must be parallel with the guide shafts 3 and 4, and a deviation of the screw shaft 5 from the parallel relationship thereof results in reduced stop position accuracy of the carriage 9.
For this reason, each part may be designed to be produced at high processing accuracy in order to maintain the above degree of parallelism. However, the control of processing and assembling such parts requires highly complicated steps. Moreover, there is some possibility of deterioration of stop position accuracy (stop position decision accuracy) of the carriage 9, even if the above degree of parallelism is maintained at the expected accuracy.
If the screw pitch of the screw portion 5a of the screw shaft 5 has a tendency to gradually increase or decrease even at minute degree, the stop position accuracy is lowered. For example, when, as shown in FIG. 7, the screw pitch of the screw portion 5a gradually decreases in the direction of increase in the track number (when P.sub.m-1 &gt;P.sub.m in the figure), the stop position decision accuracy of the carriage 9 shows a tendency to increase negative positional deviations in relation to increase in the track number, as shown in FIG. 8.
The stop position decision accuracy of the carriage 9 may be deteriorated, even if the above three shafts 3, 4 and 5 are parallel to each other and the expected pitch accuracy of the screw portion 5a of the screw shaft 5 is maintained. Since the tensile force of the springs 12 changes in relation to the stop position of the carriage 9, the vertical component Fy of the spring force F acting in the downward direction viewed in FIG. 9 decreases in accordance with the reduction of the spring force F, as shown in the same figure, when a line of action of the spring 12 is deviating from the line of axis of the screw shaft 5.
For this reason, as shown in FIG. 10, the point of contact of the pin 11 tends to rise along the rightward direction viewed in the same figure, while the spring force of the spring 12 is reduced. When the direction of decrease of the spring force corresponds to the direction of increase in the track number, each feed pitch is P.sub.n-1 &gt;P.sub.n as shown in this figure and represents the characteristic of track position decision accuracy such as shown in FIG. 8. (a dashed line in FIG. 10 indicates the position at which the pin 11 contacts the screw portion 5a by the expected pitch Po.)
It is possible to think that the deterioration in the stop position decision accuracy is caused by the action of other complex factors such as the degree of parallelism of shafts 3, 4 and 5, the pitch accuracy of the screw portion 5a, the tensile force and direction of the spring 12, inertia of the carriage 9, etc. The steps of eliminating all of these factors are laborious and require improvement in the accuracy of parts and assembling steps. There are restrictions, for example, of the direction of tensioning of the spring 12, the degree of design freedom being thereby limited.